This invention relates to the electrodeposition of gray chromium of selectively constituted crystalline character directly on aluminum base substrates and particularly to the fabrication of improved aluminum base lithographic sheet having fine secondary grained "gray" chromium directly plated thereon that can operatively function as an offset printing plate after exposure of an applied photo sensitive coating thereon.
This invention is a continuation in part of our earlier filed application Ser. No. 34,179 filed Apr. 27, 1979, now abandoned, and is particularly directed to the non-elected process subject matter disclosed and claimed therein.
Bi-metal and tri-metal lithographic plates have long been employed as an alternative to deep etch plates in the lithographic arts. As pointed out in "Chemistry of Lithography" by Hartsuch, published by the Graphic Arts Technical Foundation in 1961 (Lib of Cong. Cat. Card 60-13797) among the multimetal layered lithographic plates that have been commercially employed are the IPI tri-metal plate formed of a steel or zinc base sheet having an intermediate layer of plated copper and a surface layer of chromium plated over the copper; the "Lithure" plate formed initially of a copper sheet plated with a chromium and, more recently, of an aluminum base sheet having an intermediate layer of plated copper and a surface layer of chromium plated thereon; the "Aller" plate formed of a stainless steel base plate overlaid with plated copper; and the "Lithengrave" plate comprising a copper plated aluminum base sheet. For the purposes of convenience, both 1000 series aluminum sheet, such as 1100, and other aluminum alloy sheet used for lithographic plates, such as 3000 series sheet will be hereinafter termed generally as "aluminum" sheet or "aluminum base" sheet.
Although chromium has long been recognized as a preferred surface metal for lithographic sheet and aluminum has long been recognized as a convenient and relatively inexpensive basal sheet substrate therefor, the direct plating of gray chromium on aluminum base lithographic sheet on a commercially satisfactory basis has been a long sought but hitherto unattainable objective in the lithographic art. The patented art is replete with disclosures delineating the difficulties of directly plating chromium on aluminum or aluminum base substrates and the practical necessity of the incorporation of an intermediate coating therebetween. Whether such difficulties are attributable to the rapidity of oxide formation on aluminum surfaces or are attributable to a basic incompatability between the surface or plating adhesion characteristics of aluminum and chromium, the practical art, despite the brief appearance of chrome on aluminum lithogrpahic plates, has normally reverted back to the employment of an intermediate coating, most usually of another metal, such as Zinc, or flash coatings, such as copper, to effectively modify the aluminum base surface characteristics to permit gray chromium to be effectively plated thereon for lithographic usage.
This invention may be briefly and broadly described as an improved process for electrodepositing gray chromium of selectively constituted crystalline character and grain texture directly on aluminum base substrates. In its narrower aspects, the invention may be described as an improved process for fabricating aluminum base bi-metal lithographic plate having interfacially adherent directly plated gray chromium surface thereon of a fine secondary grain texture formed of progressively agglomerated spheroids. In a still further aspect, the invention includes the provision of an improved interfacial adherence between such electrodeposited chromium layer and an overlying coating of photo sensitive material.
Among the advantages of the subject invention is the provision of a process for the direct electrodeposition of gray chromium surface layers of fine secondary grained, closely adherent character, that are further characterized by a selectively constituted surface structure and grain texture, on base metal substrates such as aluminum. Other advantages include the provision of a chromium surfaced aluminum base lithograph plate that operatively functions, after exposure of an applied coating of photo sensitive material, as an offset printing plate; that is possessed of increased press life in terms of permitted impressions per plate, together with improved abrasion resistance, corrosion resistance and durability. Still further advantages of the subject invention are the provision of a chromium surfaced bi-metal lithographic plate that is markedly superior in photo sensitive coating adhesion, water carrying ability and printing fidelity. Additional advantages include increased efficiency of chromium plating and provision of a fine secondary grained and closely adherent directly plated chromium surface layer for lithographic plates that provide markedly improved press performance.
The primary object of this invention is the provision of an improved process of directly plating a gray chrome surface layer of markedly improved character on aluminum base lithograph sheet.
Another primary object of this invention is the provision of an improved process for obtaining gray chromium plated surfaces characterized by a selectively constituted surface structure and grain texture.